In mammals the principal ascending pathways conveying somatosensory information from peripheral receptors to the brain include: a) the dorsal column-medial lemniscal pathway, made up of large, fast-conducting primary afferent fibers originating from dorsal root ganglion neurons, and b) two pathways, the post-synaptic ascending system and spinothalamic tract, largely made up of thin, slowly-conducting fibers originating from neurons in the dorsal horn of the spinal cord. The aim of the proposed research is to explore, in the rat, the possibility that in the above ascending pathways neurons that differ as to their anatomical and functional features may be characterized by different chemical markers. In particular, the question whether aminoacidergic transmission may be a salient feature of the direct dorsal column medial lemniscal system, while the presence of high levels of neuropeptides may be a feature of ascending systems that take origin from the spinal cord, will be addressed. Primary afferent boutons of the dorsal column-medial lemniscal system ending in the cuneate nucleus will be anterogradely labeled by injection of a tracer in the dorsal root ganglia; terminals of spinal ascending systems (non-primary afferents to the cuneate nucleus and spinothalamic tract terminals to the somatosensory thalamus) will be anterogradely labeled by injection of tracer in the spinal cord. The neurochemical content of the identified synaptic terminals will be studied by means of electron microscopical immunogold labeling using specific antisera. The neuromediators that will be investigated are two excitatory aminoacids (glutamate and aspartate) and the neuropeptide substance P. Double immunolabeling experiments will also be performed to verify the possible coexistence of multiple neurotransmitters in the same identified terminal.